Rubber compositions for tire components are generally prepared in several stages. The first step is generally production of uncured rubber composition. In a rubber manufacturing plant elastomers are processed to form uncured rubber. The rubber compositions used presently in the tire industry have relatively high molecular weights and thus are highly viscous. In order to be able to handle the rubber, processing oil is added to the rubber during the work up in the rubber manufacturing plant. Said processing oil lowers the viscosity so that the compositions are more readily processable in conventional rubber processing equipment such as internal rubber mixers, open roll mixing, extrusion and calendar processing which can occur at rubber compound formulators or tire manufacturers. Said oil-extended rubber composition is dried and shaped either into particles, granules, flakes, blocks etc. for shipment to the tire manufacturer.
The second step generally includes compounding by the tire manufacturer. The oil-extended rubber composition is transported to the tire manufacturing company. The tire company combines the rubber composition with additives such as fillers, curing aids such as sulfur, activators, retarders and accelerators, processing additives such as oils, resins (e.g., tackifying resins), silicas and plasticizers, pigments, processing aids, plasticizers and reinforcing materials such as carbon black and blends the various components to prepare a compounded rubber composition: a tire rubber composition such as a rubber tread composition, an inner liner composition etc. The last step generally involves building the tire by tire manufacturer. A tire is subsequently built with the various tire rubber compositions, shaped, molded, and finally the tire is cured to obtain a tire.
As mentioned-above, rubber formulations used in various tire components previously have been designed using conventional processing oils to soften and extend the rubber. Typically, aromatic processing oils, having a certain content of polycyclic aromatic (PCA) compounds or polyaromatic hydrocarbons (PAH), have been used. See for instance U.S. Pat. No. 6,103,808 which discloses a high aromatic oil and rubber composition and oil extended synthetic rubber using the same.
In addition to its use as extender oil during rubber processing, these oils may be used in the tire industry as processing aids to ease compounding of the rubber and as plasticizers to modify the mechanical properties of the rubber compound after vulcanization. These oils may also be used to influence the visco-elastic properties of a tire compound. The aromatic content of the extender oil may enhance wet grip properties. The nature of the oils used in extending synthetic rubber and processing rubber compounds can be the same.
Mineral oils are classified by their type: Aromatic, Naphthenic and Paraffinic. Most commonly used are distillate aromatic extract (DAE) oils that are processing oils with a high polycyclic aromatic hydrocarbon (PAH) content. These DAE oils are considered to have a negative impact on the environment and human health because of the PAH content. Recently, regulatory, environmental and social concerns have necessitated the use of processing oils having a lower PAH content. As a result, there is a desire for the rubber and tire industry to cease the use of process oils that contain more than 3% DMSO Extract as determined by the test method IP346 of the Energy Institute, London. It has been estimated that the tire industry will face major challenges, as switching from DAE will require about a million metric tons of DAE to be replaced per year.
For instance U.S. Pat. No. 5,504,135 discloses an oil composition which has a kinematic viscosity at 100° C. of from 32 to 50 cSt and which contains less than 3 weight percent polynuclear aromatic compounds that is employed as a process oil for rubber compounds, especially aromatic rubbers.
US 2001/0023307 discloses a rubber process oil in which the content of polycyclic aromatics (PCAs) as determined by the IP 346 method is less than 3% by mass and which is rich in aromatic hydrocarbons, and a method for producing the same.
US 2002/0045697 which discloses a sulfur-vulcanizable rubber composition which does not contain aromatic process oils including at least one diene elastomer, at least finely dispersed, precipitated silica and carbon as fillers, softeners, at least one silane coupling agent, and additional common additives as well as a process for their production.
U.S. Pat. No. 6,399,697 which discloses a process oil which satisfies requirements that a content of a polycyclic aromatic compound is less than 3% by weight, a content of an aromatic hydrocarbon is 18% by weight or more, a content of a polar compound is between 11 and 25% by weight, a kinematic viscosity at 100° C. is between 10 and 70 mm2/s, and a flash point is 210° C. or more.
US 2005/0145312 is directed to rubber compositions for tire components. In said rubber, part of the extending oil has been replaced with soybean oil and part of the carbon black usually present in the rubber, has been replaced with a starch/plasticizer composite.
In U.S. Pat. No. 4,478,993 the use of decarboxylated rosins is disclosed for as a total or partial replacement for oil in rubber formulations.
In U.S. Pat. No. 4,515,713 decarboxylated rosin acid is disclosed that can be used as extender oil for polymers, more particularly for tire tread formulations.
WO 2002/10277 describes a butyl elastomer for use as pneumatic tire inner liners. A styrene resin is added to the butyl polymer composition to improve the green strength and the cold flow properties. Optionally up to 7 phr extender oils are added to the composition so as not to impair the air non-permeability. In the examples the extender oil has been replaced with the styrene polymer.
To replace the use of these extender oils during rubber processing the tire industry has started to use Treated Distillate Aromatic Extract (TDAE) and Mild Extract Solvate (MES) oils. Both of these oils provide tires that have better rolling resistance but, unfortunately, worse wet grip properties. Residual Aromatic Extract (RAE) oils are offered as another possible replacement, and these oils provide better wet grip properties than the other oils with at least as good rolling resistance properties.
However, in changing to the use of the lower PAH content oils some loss in rubber compound performance has been noted. In order to provide such low PAH content oils, it is therefore necessary to develop new rubber compounds that provide desirable performance levels while incorporating the use of low PAH oils.
The most commercially desirable replacement oils that meet this specification will generally provide tires with lower wet grip. Most of the new oils are still based on non-renewable mineral oils.
In order to lower the amount of non-renewable mineral oils in the final rubber composition, several other compounds were proposed to replace the processing oils that are used as processing aid in rubber compositions.
Another development in the tire rubber industry is the tendency to increase the amount of additives during compounding to compensate for the impairment of the physical properties as a result of the reduction of processing oil present or as a replacement of the processing oil. For instance the amount of Tread Enhancing Additives (TEA) during compounding is increased so as to improve the physical properties of the resulting tires such as the wet traction and rolling resistance. However, since conventional rubber compositions usually contain a certain amount of extender oils, for instance usually 37.5 phr in styrene butadiene rubber, the maximum amount of TEA that can be added to the rubber composition is restricted. Generally speaking, to an oil-extended rubber composition at most 20 phr TEA can be added during compounding.
In WO 2010/144890 tire tread compositions are disclosed wherein additives comprising unsaturated aliphatic, cyclo-aliphatic and bicyclo-aliphatic hydrocarbons, and/or olefinically unsaturated non-acidic terpene compounds, vinyl aromatic hydrocarbons and phenolic compounds are included as tread enhancing additives to a blend of oil-extended containing styrene butadiene rubber and natural rubber.
In U.S. Pat. No. 4,324,710 a high melting aliphatic hydrocarbon-insoluble resin was added to a natural rubber masterbatch together with other processing oils during compounding to form a conventional tread ply skim.
In U.S. Pat. No. 4,419,470 a low softening thermoplastic resin with low acid number was added to rubber stocks. The resin is used to replace processing oils and petroleum-derived resins normally used in rubber stocks for tire manufacture. The resin is added after the final carbon black addition, i.e. during compounding. In the examples, to a conventional natural rubber/SBR-oil extended (extended with 15 phr naphtenic process oil) rubber blend additives are compounded. One of the additives is naphtenic process oil. Half of the naphtenic process oil that is added as additive is replaced with the resin.
In U.S. Pat. No. 6,239,203 a method is disclosed to improve the abrasion resistance of a cured rubber for a tire tread compound by dispersing up to 30 phrC12-C39 alcohol and sulphur in a dry isolated sulfur-vulcanizable rubber during compounding and curing the rubber. The alcohol is used to replace the processing oil.
In U.S. Pat. No. 6,057,392 a rubber composition is described wherein 0.5 to 50 phr of a fatty acid ester is added to replace the processing oil. In the examples an oil-extended (with 37.5 phr aromatic oil) styrene/butadiene copolymer elastomer, and polyisoprene is used. The processing oil that is used during compounding is replaced with the fatty acid ester.
WO 2012/050657 describes elastomeric compositions for, for instance, tire treads and side walls. A hydrocarbon polymer modifier is added during compounding in addition to, or as a replacement for other processing aids and oils. The hydrocarbon polymer modifier is an interpolymer of at least one piperylene component, at least one cyclic pentadiene component and at least one aromatic component. It has a softening point from 80 to 160° C. a molecular weight (Mz) larger than 10.000 an at least 1 mole % aromatic hydrogen. The hydrocarbon polymer modifier is added both to oil-extended rubber and non-oil extended rubber.
In WO 2011/130525 rubber compositions are described comprising processing oil wherein the processing oil comprises tall oil pitch. It is stated that the tall oil pitch may both replace the oil present as extender in the rubber composition and as processing oil during compounding.
In WO 2015/124679 a rubber composition is disclosed wherein during compounding a combination of plasticizers is added in the form of plasticizing oil and hydrocarbon resin.
Furthermore, promising candidate TEAs often have physical properties that require the use of even more additives to make them processable. For instance promising TEAs pose dusting problems or are susceptive to remassing or sintering. In order to avoid these processing issues, additives must be used so that the maximal total amount of TEA is further lowered. There is a demand in the industry to increase the amount and type of TEA that can be present in tire rubber compositions.